WO 2004/003589 describes a seismic exploration and imaging system for producing seismic survey reports of sub-sea geological structures. In such a survey, a seismic event is applied to the seabed. The response to the seismic event in the rock of the seabed includes a compression wave (P-wave) and a shear wave (S-wave). Particles on the sea floor move in response to the P-wave and S-wave, the particles' movement being representative of the two waves. Detecting apparatus in the sea is used to monitor and record the movements of the particles, thus enabling the P-wave and S-wave to be measured and analysed. P-waves are considered well suited to imaging structures, while S-waves are well suited to determining rock and fluid characteristics.
WO 2005/078479, WO 2006/013358 and WO 2007/020396 disclose laser interferometry techniques suitable for monitoring and recording the movement of particles on the sea floor in response to the P-wave and S-wave. A plurality of interferometers may be towed on cables behind a surface vessel or submarine, the interferometers being arranged in one or more streamers. Alternatively the interferometers may be located on self-propelled vessels or submarines. In other applications, interferometers may be towed by, or located on, land vehicles or aircraft.
With conventional interferometry systems, a converging object beam is used which must be accurately focused on the surface of the object (e.g. the sea floor) being studied, or on a point below the surface of the object. Such focusing may be time consuming and may complicate the interferometry process. There is also a desire to improve the sensitivity of the interferometry system, such that finer detail may be captured. It would also be desirable to reduce problems with backscattering that are sometimes experienced with conventional interferometry systems.